Three new species of the genus Encentrum (Rotifera, Monogononta ...

Three new species of the genus Encentrum (Rotifera, Monogononta,Dicranophoridae)Willem H. De SmetSARSIADe Smet WH. 2000. Three new species of the genus Encentrum (Rotifera, Monogononta,Dicranophoridae). Sarsia 85:77-86.Three new species of Rotifera belonging to the genus Encentrum (Monogononta, Dicranophoridae)are described and illustrated. Encentrum barti sp. nov. was found among algae in the Eastern Scheldtestuary, the Netherlands. Its main distinguishing taxonomic features are the rostrum provided with atriangular projection, the conical and slightly incurved toes, and the characteristic trophi. Encentrumlistensoides sp. nov. was collected among algae in the Eastern Scheldt estuary, the Netherlands. Themain diagnostic features of this new species are the foot composed of three pseudosegments, a singlepair of large pedal glands, and the trophi showing asymmetrical unci, evenly curved manubria, andpreuncinal teeth set at a right angle to the axis. Encentrum tenuidigitatum sp. nov. was found inhygropsammon of a bight of the North Sea in Belgium. It is diagnosed by its slender, acutely pointedtoes, and the trophi bearing a tooth on the inner margin of the subbasal rami chambers.Willem H. De Smet, University of Antwerpen, R.U.C.A. campus, Department of Biology, Lab. PolarBiology and Palaeobiology, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium.E-mail: wides@ruca.ua.ac.beKeywords: Rotifera; Dicranophoridae; Encentrum sp. nov.; Belgium; Netherlands.INTRODUCTIONDetailed studies on periphytic and psammic rotifers frommarine and brackish waters are very few on a worldwidescale. Most information is to be found in Remane1949; Pennak1951; Althaus 1957a, 1957b; Fenchel &Jansson 1966; Godske Eriksen 1968; Thane-Fenchel1968; Tzschaschel 1979, 1980, 1983; Turner 1990, 1993;and Saunders-Davies 1995. With the exception of thestudies by Pennak (l.c.) and Turner (l.c.) which were donein North America, all others concern European coastalwaters. The mentioned papers also account for most ofthe rotifers known from periphytic and interstitial habitatsof the littoral zone. The rotifer fauna of these habitatsis considered impoverished relative to their freshwatercounterparts. This idea is probably correct; however,rotifer diversity is unquestionably underestimateddue to insufficient sampling efforts, preclusion becauseof the small size of the species, and difficulties in obtainingwell-preserved specimens. For the same reasonsit is likely that many unknown species await description.Dicranophoridae, and especially the genus Encentrum,are usually well represented in psammic-periphytic marineenvironments. The genus Encentrum is the mostspecies-rich of the family Dicranophoridae. In a recentrevision, De Smet 1997 distinguished 90 species, ofwhich 35 occur in brackish and marine habitats. Thepresent paper reports on three new species of Encentrumdiscovered in Belgium and the Netherlands.MATERIAL AND METHODSThe species were sampled during an ongoing study onthe rotifer fauna of the marine littoral habitats of theBelgian coast (North Sea) and the Eastern Scheldt estuary,the Netherlands.Plankton samples were collected by a horizontal haulwith a 40 µm plankton net. Hygropsammon was obtainedby scraping of the uppermost 0.5-1cm of the oxygenatedsand layer with a PVC bottle. Different species ofseaweeds were sampled separately for the study ofperiphytic rotifers. Both samples for live examination,and samples fixed with formaldehyde to a final concentrationof 4 %, were taken. The psammic and periphyticrotifers were extracted in the laboratory using the swirldecantationtechnique, and consequently concentratedon a 40 µm net.Animals were examined and drawn using a LeitzOrthoplan microscope with camera lucida. Preparationof trophi for light and scanning electron microscopy(SEM) was done following De Smet (1998) using NaOClsolution. For SEM a Philips SEM 515 microscope operatedat 20 kV was used.

78 Sarsia 85:77-86 – 2000TAXONOMIC REPORTFamily Dicranophoridae Harring, 1913Genus Encentrum Ehrenberg, 1838Encentrum barti sp. nov. (Figs 1, 2)Type locality. Eastern Scheldt, off Oesterdam, the Netherlands.20 Feb 1996.Type material. Holotype: a female in a permanent glycerineglass slide mount deposited in the KoninklijkBelgisch Instituut voor Natuurwetenschappen (KBIN),Brussels, Belgium, No. IG 28.657. Paratypes: a male andtrophi preparation in KBIN; 5 slides containing femalesin the Department of Biology, RUCA, Antwerp, Belgium;1 slide with female and trophi in the Laboratory for AnimalEcology, RUG, Gent, Belgium.Material examined. 25 females and 6 males from typelocality.Distribution and ecology. Only known from type locality.It was collected among the red alga Callithamnioncorymbosum (Smith) Lyngbye from the infralittoral zone,and constituted the main rotifer component. Co-occurringrotifers were Encentrum algente Harring, E.listensoides sp. nov., E. marinum (Dujardin), Proaleshalophila (Remane), and P. reinhardti (Ehrenberg).Etymology. The species is named after my son Bart whocollected the red algae.Female. Body (Fig. 1A, B). Body fairly stout, fusiformin dorsal view, broadest near midlength. Head c. 1/3 totallength, offset by neckfold, slightly tilted ventrally;dorsal antenna near neckfold. Rostrum short, decurvedventrally, broadly rounded with narrow triangular projectionmedially terminating in a papilliform structure.Corona oblique. Trunk without distinct folds, in lateralview dorsally arched and ventrally flattened; tail veryshort, broadly rounded; lateral antennae not seen. Footshort, conical, 2 pseudosegments. Toes (Fig. 1E, F) c. 1/7-1/8 total length, stout, tapering to acute points, basesslightly swollen, in dorsal view more or less conical,slightly incurved, in lateral view with curved dorsalmargin and almost straight ventral margin.Eyespots absent. Brain large, saccate, with Y-shapedduct; subcerebral glands present, with light-refractingglobule. Proventriculus present. Gastric glands large,rounded, apparently attached to proventriculus, no stalksseen. Two large pyriform pedal glands extending intotrunk and two small rounded pedal glands; reservoirslarge, in distal foot pseudosegment. Bladder normal.Vitellarium with 8 nuclei.Trophi of Isoencentrum-type (Figs 1G, 2A-F) accordingto De Smet (1997), relatively slender. Rami outlinemore or less heart-shaped; median rami opening broadlywedge-shaped; rami slender, each with slender, slightlyoffset and incurved apical tooth, and stout preuncinaltooth set at a more or less right angle to axis; preuncinalteeth on a level with apical teeth. Fulcrum slightly lessramus length, narrow and parallel-sided in dorsal view,in lateral view with broad base and tapering posteriorly,slightly decurved ventrally. Unci single-toothed, fairlyslender, slightly curved; tooth shaft length, strongly expandedin the horizontal plane, broadest near mid-length;ventral and especially dorsal uncinal apophyses welldeveloped; foot of shafts distinctly broadened. Intramalleistrongly flattened, tapering to acute tip inwardly; outwardlyeach with two conspicuous spiniform processes.Supramanubrium inserted on anterior margin of intramallei,narrow with pointed ends, almost straight orslightly recurved anteriorly. Manubria more or lessstraight the greatest part of their length, slightly incurvedand crutched posteriorly, head slightly expanded withtriangular expansion and large opening.Measurements. Body length 93-130 µm, toe 14-17 µm.Trophi 25.5-27.3 µm : ramus 9.1-10.8 µm, fulcrum 7.5-9.1 µm, uncus 7.9-9.9 µm, intramalleus 1.2-2.2 × 4.7-5.1 µm, supramanubrium 5.6-6.3 µm, manubrium 19.5-20.9 µm.Male. Body (Fig. 1C, D) similar to that of female, butslender and on average smaller, with narrow neckpseudosegment. Trunk narrower posteriorly with shortproximal and long distal pseudosegment. Foot present,a single pseudosegment. Penis conspicuous.Measurements. Body length 95-115 µm, toe 10 µm.Remarks. Encentrum barti sp. nov. belongs to thesubgenus Isoencentrum as defined by De Smet 1997, onbasis of the trophi morphology, especially the shape ofthe rami outline, intramallei and fulcrum, and the presenceof supramanubria.Encentrum barti sp. nov. is closely related to E. eristesHarring & Myers, 1928. It is distinguished from the latterby the presence of a triangular projection on the rostrum(absent in E. eristes), the in dorsal view more orless conical and slightly incurved toes (broadly bladeshapedand not incurved in E. eristes), the foot bearingtwo pseudosegments (a single pseudosegment in E.eristes), and its less oblique corona. Main differences introphi morphology concern the rami outline which isheart-shaped in E. barti and more elongate-pyriform inE. eristes, and the subbasal rami chambers being shorterand broader in E. barti.

De Smet – Three new species of Encentrum (Rotifera) 79Fig. 1. Encentrum barti sp. nov. A, Female, habitus, lateral view; B, Female, head and trunk, dorsal view; C, Male, habitus,lateral view; D, Male, habitus, dorsal view; E, Female, toes, lateral view; F, Same, dorsal view; G, Female, trophi, dorsal view.Scale bars A-D: 50 µm, E-G: 10 µm.Encentrum listensoides sp. nov. (Figs 3, 4)Type locality. Eastern Scheldt, tide puddle off Zierikzee,the Netherlands. 22 Feb 1995.Type material. Holotype: a female in a permanent glassslide mount deposited in the Koninklijk Belgisch Instituutvoor Natuurwetenschappen (KBIN, Brussels, Belgium,No. IG 28.657). Paratypes: a trophi preparation in KBIN;several slides containing females and trophi in the Departmentof Biology, RUCA, Antwerp, Belgium; 1 slidewith female and trophi in the Laboratory for AnimalEcology, RUG, Gent, Belgium.Material examined. 50 females from type locality. EasternScheldt, off Oesterdam, the Netherlands 3 specimens,20 Feb 1996.Distribution and ecology. So far only known from theEastern Scheldt. It was found in a temporary tide puddle

80 Sarsia 85:77-86 – 2000Fig. 2. Encentrum barti sp. nov., female. A, Trophi (SEM), ventral view; B, Same, detail dorsal view; C, Same, detail ventralview; D, Same, dorsal, postero-lateral view; E, Same, ventro-apical view; F, Same, dorso-apical view. Scale bars: 10 µm.among the tube forming colonial diatoms Parlibellusdelognei (van Heurck) Cox and P. pseudocomoides(Hendey) Cox, and in the infralittoral of the EasternScheldt among the red alga Callithamnion corymbosum(Smith) Lyngbye. Accompanying rotifers: Colurellacolurus (Ehrenberg), Encentrum algente Harring, E. bartisp. nov., E. marinum (Dujardin), Proales halophila(Remane), P. reinhardti (Ehrenberg).Etymology. The Latin suffix -oides hints at the subtledifferences between this new species and E. listenseTzschaschel, 1978.Female. Body (Figs 3A, B) somewhat elongate-fusiform,broadest at c. 1/3 from anterior margin in dorsal view;cuticle sticky, covered with more or less dense layer ofdetritus. Head c. 1/5 total length, offset by neckfold,

De Smet – Three new species of Encentrum (Rotifera) 81slightly tilted ventrally; 2 pseudosegments, anteriorsmallest, posterior with shallow dorso-lateral indentationon each side. Rostrum fairly small, broadly rounded.Corona strongly oblique. Trunk with 1-2 weak folds, inlateral view very slightly arched dorsally, highest in anteriorhalf, almost straight ventrally; tail broad; lateralantennae distally on trunk, slightly displaced dorsally.Foot short, 3 pseudosegments. Toes (Fig. 3C) c. 1/8-1/9total length, fairly slender, bases slightly swollen, graduallytapering to blunt tips, slightly decurved or kinkedventrally.Two colourless light-refracting globules near base ofrostrum. Brain relatively small, saccate, subcerebralglands present, with light-refracting globule. Proventriculuspresent. Gastric glands ovate to kidney-shaped,stalked, with connection between brain and mastax; stalksanteriorly swollen, with light-refracting globules, especiallyin distal part. Two large spherical pedals glands,connected to toes by short tubule (Fig. 3C). Vitellariumwith 8 nuclei. Subitaneous egg ovate (Fig. 3E).Trophi of Isoencentrum-type (Figs 3D, 4A-E), robust,slightly asymmetrical. Rami outline more or less oval toroughly subhexagonal; median rami opening more or lessoboval; rami stout, slightly asymmetrical, each with apicaltooth, left apical tooth stronger and more incurvedthan right; prior to apical ramus teeth a single preuncinaltooth set at right angle to axis. Fulcrum ramus length,narrow and parallel-sided in dorsal view, in lateral viewwith broad base and tapering posteriorly, slightly decurvedventrally. Unci single-toothed, stout, slightlycurved, asymmetrical, right longer and of stronger build,tooth c. 3/5-3/4 shaft length, expanded in the dorso-ventralplane, broadest near base; ventral and especiallydorsal uncinal apophyses well developed; shaft withdorsal and ventral rib. Intramallei more or less trapezoid,broader than high, bearing elongate-triangular supramanubriumon anterior margin. Manubria equally curved,crutched posteriorly, head expanded with small triangularexpansion and large opening.Measurements. Body length 132-200 µm, toe 16-19 µm.Trophi 24.2-29.6 µm : rami 8.7-9.0 & 8.9-10.8 µm, fulcrum8.1-9.5 µm, unci 5.6-6.9 & 7.5-7.6 µm, intramalleus3.3-3.5 × 5.5 µm, supramanubrium 3.8-4.4 µm, manubrium17.4-20.9 µm. Subitaneous egg 36-39 × 60-64 µm.Male. Unknown.Remarks. Encentrum listensoides sp. nov. is to be placedin the subgenus Isoencentrum by the presence ofsupramanubria, the posteriorly tapering and ventrallydecurved fulcrum, and the shape of the rami outline.Encentrum listensoides sp. nov. shows a great similarityin overall body shape with E. listense Tzschaschel,1978. Both taxa are distinguished by the number of footpseudosegments (three in E. listensoides, one in E.listense), the number and shape of the pedal glands (asingle, large pair with short ducts in E. listensoides sp.nov. and two small pairs with conspicuous ducts in E.listense) and the shape of the toes (of more slender buildand tapering more rapidly in E. listensoides). In addition,length of body and toes is greater in E. listense.There are marked differences in the trophi morphologybetween both species. E. listensoides has distinctly asymmetricalunci (symmetrical in E. listense), and preuncinalteeth set at a right angle to the axis (parallel to the apicalrami teeth and oblique to the axis in E. listense). Themanubria of E. listense are straight and posteriorlyincurved, instead of being evenly curved as in E.listensoides.The new species cannot be confused with any otherknown congener.Encentrum tenuidigitatum sp. nov. (Figs 5, 6).Type locality. Zwin Nature Reserve, located near Knokke,W. Flanders, Belgium, bight of North Sea. 14 Apr 1997.Type material. Holotype: a female in a permanent glycerineglass slide mount deposited in the KoninklijkBelgisch Instituut voor Natuurwetenschappen (KBIN),Brussels, Belgium. No. IG 28.657. Paratypes: one slidecontaining trophi preparation in KBIN; 5 slides containingfemales in the Department of Biology, RUCA, Antwerp,Belgium: 1 slide with female and trophi in theLaboratory for Animal Ecology, RUG, Gent, Belgium.Material examined. 17 females from type locality.Distribution and ecology. Only known from type locality.The new species was found in hygropsammon of thesandy beach, taken a few cm immediately above the wateredge. The accompanying rotifer fauna consisted of somebdelloid species and the monogononts Colurella colurus(Ehrenberg), Encentrum algente Harring, E. marinum(Dujardin), E. obesum Tzschaschel and Proalesreinhardti (Ehrenberg).Etymology. The new specific name tenuidigitatum (Latintenuis meaning thin, digitatum meaning provided withtoes) refers to the thin toes.Female. Body (Figs. 5A, B) fusiform in dorsal view,broadest somewhat before mid-length. Head c. 1/4-1/3total length, offset by neckfold, 2-3 weak pseudosegments;tilted ventrally; dorsal antenna near neckfold.

82 Sarsia 85:77-86 – 2000Fig. 3. Encentrum listensoides sp. nov., female. A, Habitus, lateral view; B, Habitus, dorsal view; C, Toes, lateral view; D,Trophi, ventral view; E, Subitaneous egg. Scale bars A, B: 50 µm, C-E: 10 µm.Rostrum broad, rounded. Corona strongly oblique. Trunkwith inconspicuous folds, distally slightly wrinkled, inlateral view dorsally arched with weak hump in anteriorhalf, ventrally flattened; tail very short, broadly rounded;lateral antennae prominent, c. 2/3 of the way down body.Foot short, composed of one small pseudosegment bearingtoes, and two broader and wrinkled pseudosegments.Toes (Fig. 5C, D) c. 1/7 total length, slender, tapering toacute points, bases slightly swollen, in dorsal viewslightly incurved, in lateral view more or less stronglydecurved ventrally.A single colourless light-refracting globule at base of rostrum.Brain large, saccate, subcerebral glands present, withglobule. Proventriculus present. Gastric glands large, ovateto obovate, short-stalked, with connection between brain andmastax. Pedal glands large, ovate. Vitellarium with 8 nuclei.Trophi of Encentrum-type (Figs 5E, 6A-F), robust.Rami outline longer than wide; outer margin of ramislightly concave laterally, angular posteriorly; inner marginof subbasal chambers with angular tooth; rami terminatingin a slender, offset and incurved apical toothset at almost right angle to the axis; prior to each apicaltooth ventrally a single, very stout preuncinal elementwith at right angle inwardly projecting triangular tooth;a small knob at middle of preuncinal tooth anteriorlywhereupon the ventral uncinal apophysis rests. Fulcrumlong c. 1.5 × ramus length, almost parallel-sided in dorsalview, in lateral view straight, broad at base, graduallynarrowing over c. 1/3 posteriorly and continuingparallel-sided. Unci single-toothed, slightly curved, toothc. 3/4 shaft length, more or less equally expanded allround, broadest near base; ventral uncinal apophysisweak, dorsal well developed. Intramallei trapezoid inlateral view, sock-shaped in ventral view, uncus lengthor slightly longer, opening large postero-laterally andinwardly. Manubria incus length, evenly curved, slightlytapering, crutched posteriorly, head with triangular expansiondorsally and ventrally; opening relatively small.

De Smet – Three new species of Encentrum (Rotifera) 83Fig. 4. Encentrum listensoides sp. nov., female. A, Trophi (SEM), ventral view; B, Same, detail dorsal view; C, Same, detailventral view; D, Same, detail dorso-apical view; E, Same, detail ventro-apical view. Scale bars: 10 µm.Measurements. Body length 80-127 µm; toe 13-19 µm.Trophi 23-24 µm : ramus 5.9-7.3 µm, fulcrum 8.6-9.3µm, uncus 5.0-7.1 µm, intramalleus 2.3-3.2 × 4 µm,manubrium 16.0-18.3 µm.Male. Unknown.Remarks. Encentrum tenuidigitatum sp. nov. should beplaced to the subgenus Encentrum De Smet, 1997 onbasis of the overall structure of the trophi, especially therami outline, the shape and length of the intramallei, theabsence of supramanubria, and the shape of the fulcrum.This new species can hardly be confused with any othercongener by the combination of its characteristic (1) slender,acutely pointed toes and (2) trophi bearing an angulartooth on the inner margin of the subbasal rami chambers.It superficially resembles E. eulitorale Tzschaschel,1978. However, the body of E. tenuidigitatum sp. nov. isfusiform and dorsally arched, whereas in E. eulitorale itis more or less parallel-sided and dorso-ventrally flattened.The toes of the new species are more slender andacutely tapering. E. tenuidigitatum also differs from E.eulitorale in trophi morphology. Its trophi are smaller(23-25 µm versus 40 µm in E. eulitorale), and the fulcrumis longer (1.5 × ramus length versus ramus lengthin E. eulitorale) and lacks the lateral and ventral expan-

84 Sarsia 85:77-86 – 2000Fig. 5. Encentrum tenuidigitatum sp. nov., female. A, Habitus, lateral view; B, Habitus, dorsal view; C, Toes, lateral view; D,Toes, dorsal view; E, Trophi, dorsal view. Scale bars A, B: 50 µm, C-E: 10 µm.sions at the posterior end as present in E. eulitorale. Themanubria are evenly curved and not incurved in threesteps as in E. eulitorale.The other species of the subgenus Encentrum displayinga triangular tooth on the inner margin of the rami are E.obesum Tzschaschel, 1979 and E. algente Harring, 1921.In E. obesum the body has its greatest width in the posteriorthird of the characteristic sack-shaped trunk (near mid-lengthof fusiform body in E. tenuidigitatum). Its toes are long (36µm), slightly decurved ventrally, with offset and swollenbase (toes smaller (13-19 µm), more strongly decurved, andswollen bases not offset in E. tenuidigitatum). The moreelongate intramallei and rami, with tooth on the inner marginsat 3/5 from the attachment to the fulcrum (near midlengthin E. tenuidigitatum), and the posteriorly incurvedmanubria of E. obesum (evenly curved in E. tenuidigitatum)unequivocally distinguish this species from E.tenuidigitatum.The new species is distinct from E. algente by itssmaller size and trophi length (80-127 µm versus 320-360 µm, and 23-24 µm versus 32-42 µm respectively),and especially the shape of the toes and trophi. Theslightly ventrally decurved toes of E. algente are basallyswollen and weakly offset, reduced near mid-length andcontinuing more or less tubular to the acute tip. Maindifferences in trophi morphology are found in the moreelongate rami and intramallei, the shorter fulcrum, andthe prominent tooth on the inner margin of the rami whichis lying at 3/5 from the base of the rami in E. algente.ACKNOWLEDGEMENTSI am indebted to the Department of Human Anatomy (Cell Biologyand Histology), University Centre of Antwerp for accessto the scanning electron microscope. Dr. L. Denys kindly identifiedthe diatoms. This study was partially supported by a RAFOfund awarded by RUCA.

De Smet – Three new species of Encentrum (Rotifera) 85Fig. 6. Encentrum tenuidigitatum sp. nov., female. A, Trophi (SEM), dorsal view; B, Same, latero-ventral view; C, Same, detaildorsal view; D, Same, detail ventral view; E, Same, detail dorso-apical view; F, Same, detail ventro-apical view. Scale bars: 10 µm.

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